Just bought a used Javelin rig and the leg pads aren't quite long enough to cover leg straps. Does anyone know of an easy fix without replacing the entire leg pad?

Just replace the entire legpad. They're not very expensive, and Sunpath can make you a set that will fit your thighs and match your rig. On top of that, it will make your rig more comfortable, as the padding in the legstraps breaks down after a few years, and the fresh pads will have fresh padding.

Off the top of my head, I would guess they're less than $100, and figure $20/$30 for a rigger to install.